225 N CABOT RD 2019-10-04 - Laserfiche WebLink

i Storm water run-off from the proposed duplex will be tight-lined to the west and discharge into an infiltration system. Western Washington Hydrology Model (WWHM2012) was used to analyze the size of the proposed duplex infiltration system (Refer to Section III, Duplex Infiltration Calculations). The proposed duplex infiltration system will be 25 ft x 10 ft, as shown on the construction plans and will fully infiltrate storm water run-off into the existing ground. The proposed driveway access is classified as pollution generating impervious surface. The proposed asphalt driveway will take access from North Cabot Road (See Figure 3). The proposed asphalt will consist of two driveways: 515 sf and 625 sf, Unit A and Unit B respectfully. Storm water run-off from Unit A's driveway will sheet flow to the west through a 2 ft transition zone and continue to flow through native vegetation a minimum of 25 ft, from the discharge point to the nearest property line. Storm water run-off from Unit B's driveway will sheet flow to the west and discharge into an infiltration system located between the proposed accesses. WWHM2012 was used to analyze the size of Unit B driveway infiltration system (Refer to Section III, Unit B Infiltration Calculations). The proposed Unit B driveway infiltration system will be 20 ft x 2 ft, as shown on the construction plans and will fully infiltrate storm water run- off into the existing ground. WWHM2012 was used to calculate the volume of storm water run-off from the site for existing and developed conditions. The project site has been modeled as forested land cover for pre- developed conditions. The project site has been divided into several different land cover types, for modeling purposes, for mitigated conditions (Figure 4: On-Site Basin Map). The proposed developed will not increase storm water run-off more than 0.15 CFS, for the pre-developed to developed conditions, for the 100 year return period storm event (Refer to Section III, Site Calculations). All on-site disturbed areas, not covered with impervious surface, will be amended with existing top soil in accordance with SMMWW Volume V, BMP T5.13. Due to the type of soils and vegetation on site, the distance from the developed area to the property line, and the BPM's used on this proposed project, it is expected to meet the intent to provide both flow control and storm water treatment. OFF-SITE ANALYSIS Upstream Analysis: Based on, on-site field observations and county contour maps, there is a small upstream basin area, approximately 1 acre, located to the west of the parcel (Figure 5: Off-Site Drainage Map). The upstream basin area consists of developed SFR lots and road systems. Due to the size of the upstream basin area, together with the length of flow path and the types of soils,the upstream basin will not negatively impact the proposed project. 3